1. Field of the Invention
The present invention relates to an apparatus and method for automatically controlling a gain of a signal in a portable communication system. More particularly, the present invention relates to an apparatus and method for predicting a head room size depending on a channel variation of a time axis and for adjusting the total dynamic range of an Analog to Digital Converter (ADC), thus improving the performance of a receive apparatus in a portable communication system.
2. Description of the Related Art
A wireless communication system is constructed to vary an amount of amplification of a signal depending on a distance and channel environment between a transmit apparatus and a receive apparatus and transmit the signal. If the wireless communication system does not use a suitable amount of amplification for the distance and channel environment between the transmit apparatus and the receive apparatus, the wireless communication system may fail to receive the signal or may cause heavy interference in other Mobile Stations (MSs).
Thus, when transmitting a wireless signal as above, transmitting the wireless signal using too much power of a specific signal has a negative influence on other MSs. That is why the receive apparatus requires a scheme of suitably amplifying a received signal. Thus, the receive apparatus receiving wireless signals has amplifiers for amplifying the received signals.
In addition, when a wireless signal is transmitted as is, it may cause a waste of a bandwidth, a data error, and the like. In order to avoid this problem, the wireless communication system uses a preset coding and modulation scheme for data transmission/reception.
If using the preset coding and modulation scheme as above, the wireless communication system may perform modulation and coding using a specific modem chip device and, in the receive apparatus, perform demodulation and decoding using the modem chip device.
The above automatic gain control process of the portable communication system accumulates a spontaneous power of a signal sampled on a time axis, obtains an average power at a preset time, compares an input reference power value of an Analog to Digital Converter (ADC), and controls a gain amplifier of an input stage of the ADC. At this time, the reference power value of the ADC refers to a Root Mean Square (RMS) of data to be demodulated by an MS.
Thus, the receive apparatus fixes a reference input range for data demodulation at the total input level fixed and a fading margin to overcome a channel variation and fixes the maximum Signal to Noise Ratio (SNR) of an input signal of the ADC to a reference input valid bit.
Thus, the maximum operation region (i.e., dynamic range) of the ADC includes a reference input enabling the receive apparatus to express the required maximum Signal to Quantization Noise power Ratio (SQNR) and a fading margin enabling toleration of the maximum channel variation.
However, the maximum channel variation and maximum SNR required by the receive apparatus form a reciprocity relationship and thus, there is no case where the fixed maximum operation region (i.e., dynamic range) of the ADC is used in all circumstances. Therefore, the above case has a problem of deteriorating the performance of the receive apparatus.